Fesoterodine fumarate is the international nonproprietary name (INN) of the active principle 2-((R)-3-diisopropylammonium-1-phenylpropyl)-4-(hydroxymethyl)phenylisobutyrate hydrogen fumarate, whose structure formula is reported herein below.

Fesoterodine fumarate was approved in Europe and in the U.S.A. for the treatment of overactive bladder syndrome with the commercial name of TOVIAZ®.
Fesoterodine fumarate was described for the first time in U.S. Pat. No. 6,858,650, which reports the preparation of the active ingredient for the salification of fesoterodine with fumaric acid, according to scheme 1 reported below.

In turn, fesoterodine (also called fesoterodine base) is described in U.S. Pat. No. 6,713,464, where it is prepared starting from a deacylated precursor, (R)-feso deacyl, i.e. (R)-2-[3-(diisopropylamino)-1-phenylpropyl]-4-(hydroxymethyl)phenol, according to scheme 2 reported herein below.

The preparation of (R)-feso deacyl is instead described in U.S. Pat. No. 5,559,269.
As inferable from scheme 2, the transformation of (R)-feso deacyl into fesoterodine provides for the selective esterification of a phenolic hydroxyl in presence of an alcoholic hydroxyl; in the prior art (U.S. Pat. No. 6,713,464 and U.S. Pat. No. 6,858,650) such selective esterification is carried out at 0° C., with an exactly stoichiometric quantity of esterifying agent (isobutyryl chloride) and triethylamine, as scavenger of HCl generated by the esterification reaction. Fesoterodine thus obtained normally contains two typical impurities, i.e. the diesterification product, whose chemical name is (R)-2-[3-(diisopropylamino)-1-phenylpropyl]-4-isobutyroyloxymethyl-phenyl isobutyrate; and the starting product, i.e. (R)-feso deacyl.
WO2007140986 describes an improved esterification method with respect to U.S. Pat. No. 6,713,464 in which, in place of triethylamine, diisopropylethylamine (Hünig base) is used, which allows to improve the esterification selectivity. From table 1 of WO2007140986, it is possible to observe however that, even in the best proposed conditions (example 2), a diester quantity equal to 1.73% is formed, while in the conditions suggested by U.S. Pat. No. 6,713,464 (example 3) 5.90% diester is formed, and in absence of HCl scavenger bases (example 4) 2.90% diester is formed, while the starting product (Metabolite) remains non-reacted in a percentage equal to 2.9%.
There is therefore the need for a selective acylation method of (R)-feso deacyl which leads to fesoterodine (and thus to fesoterodine fumarate) with high yields and with a greater degree of purity, and in particular with a lower diester and (R)-feso deacyl content.